1. Field of Invention
The present invention pertains to the field of magnetic memories. More particularly, this invention relates to a an improved reference layer structure in a magnetic storage cell.
2. Art Background
A magnetic memory such as a magnetic random access memory (MRAM) typically includes one or more magnetic storage cells. Each magnetic storage cell usually includes an active layer and a reference layer. The active layer is usually a layer of magnetic material that stores magnetization patterns in orientations that may be altered by the application of magnetic switching fields. The reference layer is usually a layer of magnetic material in which magnetization is fixed or "pinned" in a particular direction.
The logic state of such a magnetic storage cell typically depends on its resistance to electrical current flow. Its resistance usually depends on the relative orientations of magnetization in its active and reference layers. A magnetic storage cell is typically in a low resistance state if the overall orientation of magnetization in its active layer is parallel to the orientation of magnetization in its reference layer. In contrast, a magnetic storage cell is typically in a high resistance state if the overall orientation of magnetization in its active layer is anti-parallel to the orientation of magnetization in its reference layer. Such a magnetic storage cell is usually written to a desired logic state by applying magnetic switching fields that rotate the orientation of magnetization in its active layer. It is usually desirable that a magnetic switching field of a predictable magnitude in one direction switch a magnetic storage cell to its low resistance state and a magnetic switching field of the same predictable magnitude in the opposite direction switch the magnetic storage cell to its high resistance state. Such switching behavior may be referred to as symmetric switching characteristics. Unfortunately, a variety of effects commonly found in prior magnetic storage cells may disrupt magnetization in an active layer and create asymmetric switching characteristics.
For example, the reference layer in a typical prior magnetic storage cell generates demagnetization fields that push the magnetization in the active a layer toward the anti-parallel orientation. These demagnetization fields usually increase the threshold magnitude of the magnetic switching field needed to rotate the active layer to the low resistance state and decrease the threshold magnitude of the magnetic switching field needed to rotate the active layer to the high resistance state. This typically increases the power needed to write the magnetic storage cell to the low resistance state and may cause accidental writing to the high resistance state. In extreme cases, these demagnetization fields may cause a magnetic storage cell to remain in the high resistance state regardless of the applied magnetic switching fields history.
In addition, coupling fields between the reference layer and the active layer in a prior magnetic storage cell usually push the magnetization in its active layer toward the parallel orientation. These coupling fields usually increase the power needed to write a magnetic storage cell to the high resistance state and may cause accidental writing to the low resistance state. In extreme cases, these coupling fields may cause a magnetic storage cell to remain in the low resistance state regardless of the applied magnetic switching fields history.
Moreover, the degree of disruption to the magnetization in an active layer caused by demagnetization and coupling fields may vary among the magnetic storage cells in an MRAM array. In addition, such disruptions may vary between different MRAM arrays due to variation in the patterning steps and/or deposition steps of device manufacture. Such variations typically produces uncertainty as to the behavior of individual magnetic storage cells during write operations.